LTC7813 datasheet(13/38 Pages) LINER

13 / 38 page

LTC3895

3895fa

For more information www.linear.com/LTC3895

operaTion

The RUN pin has no internal pull-up current, so the pin

must be externally pulled up or driven directly by logic.

The RUN pin can tolerate up to 150V (absolute maximum),

so it can be conveniently tied to VIN in always-on applica-

tions where the controller is enabled continuously and

never shut down.

The start-up of the controller’s output voltage VOUT is

controlled by the voltage on the SS pin. When the voltage

on the SS pin is less than the 0.8V internal reference, the

LTC3895 regulates the VFB voltage to the SS pin voltage

instead of the 0.8V reference. This allows the SS pin to

be used to program a soft-start by connecting an exter-

nal capacitor from the SS pin to GND. An internal 10μA

pull-up current charges this capacitor creating a voltage

ramp on the SS pin. As the SS voltage rises linearly from

0V to 0.8V (and beyond), the output voltage VOUT rises

smoothly from zero to its final value.

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Forced Continuous Mode) (MODE Pin)

The LTC3895 can be enabled to enter high efficiency

Burst Mode operation, constant frequency pulse-skipping

mode, or forced continuous conduction mode at light load

currents. To select Burst Mode operation, tie the MODE

pin to GND or a voltage between 0.5V and 1.0V. To select

forced continuous operation, tie the MODE pin to INTVCC.

To select pulse-skipping mode, tie the MODE pin to a DC

voltage greater than 1.4V and less than INTVCC – 1.3V.

This can be done with a simple resistor divider off INTVCC,

with both resistors being 100k.

When the controller is enabled for Burst Mode operation,

the minimum peak current in the inductor (burst clamp) is

adjustable and can be programmed by the voltage on the

MODE pin. Tying the MODE pin to GND sets the default

burst clamp to approximately 25% of the maximum sense

voltage even when the voltage on the ITH pin indicates a

lowervalue.Avoltagebetween0.5Vand1.0VontheMODE

pin programs the burst clamp linearly between 10% and

60% of the maximum sense voltage.

In Burst Mode operation, if the average inductor current

is higher than the load current, the error amplifier, EA, will

decrease the voltage on the ITH pin. When the ITH volt-

age drops below 0.425V, the internal sleep signal goes

high (enabling sleep mode) and both external MOSFETs

are turned off. The ITH pin is then disconnected from the

output of the EA and parked at 0.450V.

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current that the LTC3895 draws

to only 40μA. In sleep mode, the load current is supplied

by the output capacitor. As the output voltage decreases,

the EA’s output begins to rise. When the output voltage

drops enough, the ITH pin is reconnected to the output

of the EA, the sleep signal goes low, and the controller

resumes normal operation by turning on the top external

MOSFET on the next cycle of the internal oscillator.

When the controller is enabled for Burst Mode operation,

the inductor current is not allowed to reverse. The reverse

current comparator (IR) turns off the bottom external

MOSFET just before the inductor current reaches zero,

preventing it from reversing and going negative. Thus,

the controller operates discontinuously.

In forced continuous operation, the inductor current is

allowed to reverse at light loads or under large transient

conditions. The peak inductor current is determined by

the voltage on the ITH pin, just as in normal operation.

In this mode, the efficiency at light loads is lower than in

Burst Mode operation. However, continuous operation

has the advantage of lower output voltage ripple and less

interference to audio circuitry. In forced continuous mode,

the output ripple is independent of load current.

WhentheMODEpinisconnectedforpulse-skippingmode,

the LTC3895 operates in PWM pulse-skipping mode at

light loads. In this mode, constant frequency operation

is maintained down to approximately 1% of designed

maximum output current. At very light loads, the current

comparator, ICMP, may remain tripped for several cycles

and force the external top MOSFET to stay off for the same

number of cycles (i.e., skipping pulses). The inductor cur-

rent is not allowed to reverse (discontinuous operation).

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